1. Field of the Invention
The invention relates to a control system for an automatic transmission.
2. Related Art
In the automatic transmission of the prior art, rotation generated by the engine is transmitted through a fluid transmission unit such as a torque converter to a speed change unit so that a speed change may be effected in the speed change unit. Between the fluid transmission unit and the speed change unit, moreover, there is arranged a first clutch (or an input clutch) which can be applied and released to interchange ranges between a neutral range (hereinafter "N-range") and a forward running range (hereinafter "D-range").
In an automatic transmission, moreover, neutral control is executed when the vehicle is stopped with the D-range being selected, to place the first clutch in the state which immediately precedes the start of its application, so that in starting forward movement of the vehicle the load applied to the engine may be reduced to improve the fuel economy while preventing shock.
In neutral control, the neutral state of the first clutch is established by raising/lowering the oil pressure fed to the hydraulic servo of the first clutch, stepwise with a predetermined oil pressure increment in a manner to correspond to the change in the difference between the input side RPM and the output side RPM of the fluid transmission unit, e.g. a torque converter. Also executed (as disclosed in Japanese Patent Laid-Open No. 293687/1995) are: a release control for establishing the neutral control by lowering the oil pressure in the hydraulic servo, gradually, from the state in which the first clutch is applied; and an in-neutral control for maintaining the neutral control.
However, the automatic transmission control system of the prior art described above has failed to provide for stable execution of the release control and the in-neutral control.
If the oil pressure increment (of change) is enlarged in the case of a fluid transmission unit having a large stall coefficient and a high stall torque, the rotational speed difference becomes so excessive that the first clutch is released too much to allow the release control to stabilize.
If, on the other hand the oil pressure increment is reduced, the rotational speed difference (between fluid transmission input and output) is reduced to prevent the excessive release of the first clutch in the release control. However, at a low oil temperature, the rotational speed difference is so small that the first clutch cannot be returned to the state which immediately precedes the start of its application and stable in-neutral control cannot be provided.